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Structure, mechanism and regulation of the V-ATPases

Michael D Forgac

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National Institutes of Health (NIH)
The long term goals of this research are to determine the mechanism and regulation of the vacuolar (H+)- ATPases (V-ATPases). The V-ATPases are ATP-dependent proton pumps that function in both normal and disease processes, including membrane traffic, viral infection, urinary acidification, bone resorption and tumor invasion. V-ATPases are multisubunit complexes composed of a peripheral V1 domain that hydrolyzes ATP and an integral V0 domain that translocates protons. V-ATPases are regulated in vivo by reversible dissociation of the V1 and V0 domains. The first objective of this proposal is to test the role of helical swiveling within the V0 domain in proton transport. We have obtained evidence for helical swiveling (rotation of a helix about its long axis) in both subunit a and the proteolipid subunits. We hypothesize that swiveling of helices containing transport critical residues functions in proton translocation through V0. The second objective of this proposal is to elucidate the mechanism by which glucose regulates V-ATPase assembly in yeast. We have developed a novel genetic screen for regulators of V-ATPase assembly and used this screen to identify the Ras/cAMP/PKA pathway as a key regulator. We hypothesize that there are novel regulators in addition to PKA that control V-ATPase assembly, and have recently identified protein phosphatase PP1 as one such regulator. We will test these hypotheses and achieve our objectives by pursuing the following Specific Aims. Specific Aim 1 - To test the hypothesis that helical swiveling within the V0 domain functions in proton transport, we will determine the effect of intramolecular, disulfide-mediated cross-linking between adjacent helices within both subunit a and subunit c' on proton transport activity. We expect that if helical swiveling is required for proton transport, preventing helical swiveling by cross-linking adjacent helices within subunit a or subunit c' will inhibit activity. Specific Aim 2 - To determine the mechanism by which glucose regulates V-ATPase assembly in yeast, we will identify and characterize additional novel regulators of V-ATPase assembly using our modified genetic screen. These regulators include both essential and non-essential genes whose mutation blocks V- ATPase dissociation or reverses PKA-mediated assembly. The proposed research is significant because it will greatly advance our understanding of the mechanism by which V-ATPases carry out proton transport and facilitate the identification of novel regulators of V-ATPase assembly. Because V-ATPases are highly conserved between yeast and mammalian cells, as is the use of regulated assembly to control V-ATPase activity, and because regulators such as glucose, PKA and aldolase control assembly in both yeast and higher eukaryotes, these studies will likely provide important insight into control of V-ATPase assembly in mammalian systems. These insights will in turn facilitate the development of therapies to modulate V-ATPase activity that could prove effective in the treatment of diseases, such as viral infection, osteoporosis and cancer, in which V- ATPases participate.

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